1. Technical Field
This invention is concerned with the design of a lightweight, durable glue-on horseshoe.
2. Related Art
The evolution of the present day plastic glue-on horseshoe is traceable through the references cited. A glue-on horseshoe is one that is attached to the hoof by means of an adhesive only; that is without the use of nails or screws. As discussed in many of the references, the elimination of nails is a very healthful development for hoofs. The art related to glue-on horseshoes involves adhesives, materials of construction, contours, strengthening means, fitting, resistance to lateral loads, and wear.
The driving factor in the development of the glue-on horseshoe has been finding a satisfactory adhesive. This problem is very difficult, principally for the following reasons: the impact loads produced when the hoof of a running horse strikes the ground are very large (several thousand pounds); both the hoof and the plastic shoe offer surfaces that tend to be difficult for bonding; and the adhesive must set within a minute or so because of horses' impatience. These factors, taken together, preclude the use of many of the common adhesives.
The adhesive utilized in the two principal glue-on shoes presently marketed in the U.S., by Mustad (patent pending) and Cavallo (under Tovim's British Patent No. 1517113 which is a precursor to Tovim's referenced U.S. patents), both use a cyanoacrylate adhesive. Also a cyanoacrylate, or its generic descriptor "quick setting adhesive", is specified in a number of the referenced U.S. patents including Thoman, Tovim, and Battista. The other common adhesives including hot melts, epoxies, acrylics, and urethanes have very little constituency compared to cyanoacrylates, probably because experience has shown that they will not perform satisfactorily.
Cyanoacrylates have a negative feature for horseshoe applications--they have little gap-filling capability. The horseshoe must be designed to accommodate this limitation. Very close-mating surfaces are required. Consequently the shoe parts being glued to the sidewall of the hoof must be flexible so that they can be pressed close against the surface. Also gluing is not effective on the bottom of the hoof because it is too uneven.
The plastic material most often used for horseshoes is polyurethane. It is specified in most recent patents and is used in both of the commercially available horseshoes previously cited. Polyurethanes can be grouped into three general categories: foamed; flexible (characterized by hardnesses on the Shore A scale); and semihard (characterized by hardnesses on the Shore D scale). Foamed polyurethanes are too weak for structural use in a horseshoe. Flexible polyurethanes are suitable for horseshoes bonded with cyanoacrylates. Semihard polyurethanes have apparently not heretofore been utilized. An exception to polyurethanes is the recent referenced patent of Battista which specifies a horseshoe molded from polyphenylene sulfide resin.
Virtually all of the horseshoes cited have upward-extending member(s) for attachment to the sidewall of the hoof. The upward-exte nding members often take the form of a plurality of tabs, per Mustad and Cavallo. The upward-extending member(s) are generally contoured to the local angle of the hoof sidewall, per Mustad, Cavallo, Thoman, Cameron, Tovim, Dallmer, and Renkenberger et al.
The bottom part of a polyurethane horseshoe alone is generally not strong enough to withstand the large bending moments to which it may be subjected. Moreover no added strength is provided by the top part when using cyanoacrylate adhesives, because the upward-extending member(s) must be flexible. To provide the additional strength as well as to aid in fitting, an adjunct metal member is often incorporated into the plastic bottom part of the shoe, per Cameron, Tovim, Dallmer, and MacGuire-Cooper. In the horseshoe marketed by Mustad the adjunct member is actually a standard aluminum shoe either imbedded in, or attached to the polyurethane bottom part.
If the horseshoe contains an adjunct metal member, the shoe is fitted to the generally U-shaped individual hoof by cold forming the metal member, per Cameron, Dallmer, and Mustad. If the shoe is all plastic, it is can be supplied oversized and then trimmed to fit the individual horse, per Tovim, or it can be thermofitted, as per Thoman for his polyurethane horseshoe and Battista for his polyphenylene sulfide resin horseshoe.
Glue-on horseshoes, particularly those without metal strengthening members, need special features to resist the sidewall debonding that can result from lateral relative movement of the shoe with respect to the hoof, as tends to occur, for example, during pivoting. Often a crossbar joining the heels of the shoe is specified per Tovim, Dallmer, McDonnell, and MacGuire-Cooper. However, in practice, crossbars are not very effective in preventing sidewall debonding. Instead of a crossbar, Spencer uses two cantilevered bars extending inward to the sulci with the bars terminating in upward projections shaped by the farrier to fit into the individual sulcus. Thoman solves the problem in a different manner by cutting a ridge along the periphery of the hoof bottom for positive mating into a recess formed in the corresponding part of the shoe.
Virtually all of the cited glue-on horseshoes have the disadvantage that, when used with active horses on hard surfaces like dirt as opposed to soft surfaces like grass, they wear much more rapidly than do conventional steel horseshoes. If the bottom surface of the shoe is plastic, the wear problem is generally severe. The shoes by Cameron, Tovim, Dallmer, and Renkenberger et al and some versions by Thoman and Mustad fall into this category. However other versions by Dallmer, Thoman and Mustad have bottom surfaces of aluminum or even steel. Attaching the metal bottom to the plastic base of the shoe presents a special problem. The commercial version of Dallmer's shoe uses rivets. Mustad claims to have solved the problem of gluing aluminum to polyurethane in the manufacturing process for the racing version of their shoe. Thoman proposes to glue a metal bottom to his plastic shoe after it has been bonded to the hoof; however there is available to the farrier no bonding process that will maintain its integrity for very long under the loads to which the hoof is subjected, and Thoman gives no indication of how to solve this problem. A novel solution to the wear problem was proposed by MacGuire-Cooper wherein he specified the addition of abrasive particles to his plastic shoe in order to increase wear resistance. Unfortunately plastics cannot retain abrasive particles under the loads commonly encountered when the hoof strikes the ground.
The previous discussion has summarized the related art prior to the impact of my new process for bonding a polyurethane horseshoe, titled "Process for Gluing to a Horse's Hoof", U.S. Pat. No. 5,069,289, filed on Jun. 18, 1990. Therein a process is disclosed whereby a secure bond is produced by preheating both the hoof and the polyurethane shoe to about 275.degree. F. prior to applying a selected urethane paste adhesive. The adhesive is gap-filling and sets in about one minute. The present application discloses an optimum horseshoe to exploit the advantages inherent in this bonding process.